1. Field of the Invention
The present invention relates to a player of a recording medium for playing optical recording medium such as an optical disk where an information signal, e.g. an image signal, is recorded and for reproducing the information signal, and in particular to a player of optical recording medium, which can play both media carrying information at a high recording density and at a low recording density, respectively
2. Description of the Prior Art
As the light source for the optical recording medium player, a semiconductor laser is often used. At present, a semiconductor laser emitting near infrared light with wavelength of 780 nm is practically used as the light source for such optical recording medium player.
With the rapid development in a laser light source in recent years such as short wavelength semiconductor laser or laser source utilizing optical non-linearity, it is now becoming possible to use a laser light source, which emits a red light with wavelength of 670 nm, a green light with wavelength of 532 nm or a blue light with wavelength of 430 nm. If the information signal is recorded on an optical recording medium while using laser light source with such a short wavelength, it is possible to record an ultra-fine image signal or to miniaturize the optical recording medium. In this connection, new standards have been proposed using these types of laser.
In general, when a laser beam is irradiated on an optical recording medium, the spot diameter of the light on the recording medium is given by EQU s=.lambda./NA,
where .lambda. represents a wavelength, and NA is a numerical aperture of an objective lens. That is, the shorter the wavelength is, the smaller the spot diameter can be. As a result, the pit length d1 of the medium formed by the light source with a short wavelength of .lambda. 1 can be reduced down to about s1 (=.lambda. 1/NA), and it can be approximated as follows: EQU dl=.lambda. 1/NA
The pit width is also approximately equalized to s1, and the gap between the adjacent tracks can be narrowed down. Thus, it is possible to manufacture an optical recording medium of a high recording density.
If a recording medium with a low recording density recorded by light source with near infrared laser wavelength .lambda. 0 can be read by the light source with short wavelength .lambda. 1 (.lambda. 0&gt;.lambda. 1), it is preferable because this can be used for playing the media of both high and low recording densities. The pit length d0 of the medium of low recording density is approximatly equal to the irradiation spot diameter s0 of the laser beam with wavelength .lambda. 0, and it can be approximated as follows: EQU d0=0/NA
On the other hand, the spot diameter s1 of the laser beam to read this is based on the relation d0&gt;s1 from the relation of .lambda. 0&gt;.lambda. 1. Thus, the irradiation light spot diameter of high recording density is smaller than the pit length of the disk of low recording density. However, even in this case, it is possible to read the recorded signal on the medium, and the pit of the medium is of such structure, in the combination of a numerical aperture NA of an objective lens and the wavelength .lambda., as to minimize the generation of noise beam, which is the so-called intermodulation. When, as a result, the medium carrying information recorded to be read by means of a light beam with a wavelength .lambda. 0 is read by a different light beam with a short wavelength .lambda. 1, intermodulation occurs during the reading operation.